STOP-ROP is a multicenter randomized comparison of supplemental therapeutic oxygen and conventional oxygen therapy for treatment of prethreshold retinopathy of prematurity. The purpose of this study is to determine if supplemental therapeutic oxygen will reduce the proportion of infants with prethreshold ROP who advance to threshold ROP. Both treatment groups will be continuously monitored via a study pulse oximeter and a bedside computer until both eyes have met either regression or progression to endpoint. Final opthalmology exam is at 3 months' corrected gestational age.